Semiconductor dice are normally packaged in a plastic capsule to protect them from physical damage. Connections are made to external circuitry, such as a printed circuit board (PCB), by means of sheet metal pieces, called “lead fingers,” that protrude from the capsule and can be soldered or otherwise connected electrically. Alternatively, in a “no lead” package the external connections are made in other ways, e.g., by means of contacts having faces that are flush with the surfaces of the capsule. Two types of “no-lead” packages are designated in the industry as QFN (Quad Flat No Lead) and DFN (Dual Flat No Lead).
A cross-sectional view of a typical “no-lead” package 10 is illustrated in FIG. 1A. A bottom view of the package is shown in FIG. 1B. Package 10 contains a semiconductor die 12, a die-attach pad 14, bonding wires 16 and contacts 18, through which die 12 may be contacted electrically. Die 12 is bonded to die-attach pad 14 with an adhesive layer 20, which may be epoxy. These components are encased in a capsule 22 which is made of a plastic molding compound. Package 10 may be mounted on a PCB, with the bottom surfaces of contacts 18 soldered, for example, to metal pads or traces on the PCB.
“No-lead” packages are normally manufactured by attaching a plurality of dice to corresponding die-attach pads in a leadframe. The dice and die-attach pads are then encased in a plastic molding compound, and the dice are separated by sawing to produce individual packages each containing a single die.
In practice, the packages must satisfy a wide variety of constraints and requirements that are not met by the basic package shown in FIGS. 1A and 1B. For example, in some situations the designer of the equipment in which the package will be used wishes to have exposed traces on a PCB underneath the package, in which case the die-attach pad of the kind shown in FIGS. 1A and 1B would create short-circuits between the traces. In other cases the equipment designer wishes to include several dice or a single large die within the package, in which case a package of the kind shown in FIGS. 1A and 1B might be too thick. In some cases, it is desired to have a die-attach pad exposed on one principal surface of the package and contacts exposed on the opposite principal surface of the package. Another problem that can occur is short-circuiting resulting from solder bridging between the terminals or between a terminal and an exposed pad.